Thermosets

Thermosets are a class of polymers that irreversibly set or cure to form a rigid, three‑dimensional network. Unlike thermoplastics, which soften and can be remolded when heated, thermosets do not melt once cured due to extensive crosslinking.

Processing involves mixing a resin with a curing agent, catalyst, or hardener and then heating or exposing to other triggers to initiate a chemical reaction. The curing forms covalent bonds between polymer chains, creating a solid crosslinked network whose properties depend on crosslink density and the chemistry used.

Common families include epoxy resins, phenolic resins (such as novolaks and resols), unsaturated polyesters, polyurethane resins, urea-formaldehyde and melamine-formaldehyde resins, and silicone-based elastomers. Epoxies are widely used as adhesives and protective coatings; phenolics as heat-resistant binders; polyurethanes in foams and coatings; unsaturated polyesters in composites; silicones in seals and gasketing.

Thermosets typically exhibit high thermal stability, chemical resistance, hardness, and dimensional stability, along with good electrical insulation. They tend to be more brittle than many thermoplastics and have limited or no reworkability after curing.

Applications include electronic encapsulation, coatings, adhesive bonds, fiber-reinforced composites, and molded parts in automotive and industrial equipment. A key limitation is recyclability; after cure they cannot be remelted, so recovery generally relies on mechanical grinding, chemical recycling, or reuse as fillers. Advances aim to improve toughness and recyclability, including approaches with reversible crosslinks or bio-based monomers.